To make it more re-playable.
Make your idea challenge mode. With lots of different challenges.
Earn X by this year. Earn X profit per month/yr.
passengers per. 30+ challenges. Each 1 progressively harder.

Campaign mode should be more like starting in Jan. 1, 1914, the St. Petersburg-Tampa Airboat Line became the world’s first scheduled passenger airline service, operating between St. Petersburg and Tampa, Fla.

With very little help from the game in the form of technology.
The flight above lasted only 4 months.
Was way to costly.

Over the years you have to keep upgrading your terminal to keep up with the demand from airlines. Otherwise they will go somewhere else.
All the way until 2095 and your launching space flights from your terminal.

What about starting the airport some time in the earlier times of aviation, say 1920 or so. You would only be able to build a simpler (maybe even grass) and some hangars. No complicated check-in or anything. Then you would start unlocking objects and building types as time progresses.
Time would also have an effect on the types of planes arriving at your airport. Initially you would only get prop planes and eventually progress to jets.
I think something like this would add a lot more durabi…